This invention relates to apparatus and methods for controlling the current in a resonant secondary or pickup circuit for an inductive power transfer circuit, capable of collecting electricity from a primary distribution pathway having one or more conductors carrying alternating current.
In present day inductive power transfer (IPT) systems there is an energisable trackway having at least one conductor (the primary), each of which is surrounded by an alternating magnetic field during use One or more pickup devices, each of which includes at least one pickup winding, form part of the system. Each is placed so as to intercept a part of the alternating magnetic field of the primary and thereby induce a useful current in the winding. Usually, the frequency of the alternating current in the primary is more or less matched to a resonant frequency within the pickup. Practical supply frequencies range from a mains frequency (50 Hz) up to commonly used frequencies in the 5-56 kHz range, and as components capable of handling high power at higher frequencies become available, the usable frequency may become higher Generally, the supply frequency should be stable.
Resonant pickups may be either parallel-tuned or series-tuned in order to improve the transfer of power Control of the power picked up from an IPT system has been a problem
The problem of control of power transfer might be solved by setting up a system with a high capacity if uncontrolled and then either decoupling the link between the primary pathway and the secondary pickup in some way, or by xe2x80x9cwastingxe2x80x9d excess power within the pickup circuit
Decoupling by interfering with the magnetic circuit itself could be done by altering one or more of the dimensions of the gap, by adding or subtracting permeable materials, by introducing a conductive block in which eddy currents may be generated, or (as a passive over-supply limit) by incorporating a saturable ferri- or ferromagnetic element into the magnetic circuit.
A related form of decoupling comprises changing the resonant frequency of (usually) the pickup. Because this form can settle to a stable frequency if the supply and load powers are stable, we regard it as a T=infinity configuration.
Many plans for IPT systems existed in the latter 19th century; for example Tesla held a patent for powering a train using a high-voltage system with capacitative coupling, and a number of inventors filed patents for at least telegraph message transfer by inductive means across a wide gap from a moving railway triage to a stationary trackway.
In the 20th century there were many attempts to make commercial use of IPT systems, perhaps the most successful of these for larger power applications (e.g. to moving vehicles) is that of the Boys (the present inventor).
Otto. GB1418128 (December 1974), described a series-tuned pickup having a capacity suitable for use in powering a bus. Control of the power picked up was not included. Boys et al, in U.S. Pat. No. 5,293,308 disclosed a parallel-tuned pickup control.
The problem to be solved is, to provide control over the transfer of inductive power into any one pickup device to be at a level that matches the power being consumed If the transferred power is too small the load is starved. If the transferred power is too large, the surplus current circulates within the pickup or over-supplies the load and may cause damage. Furthermore, surplus circulating current, by generating its own field, can block the onwards passage of primary power to other secondary circuits sharing the same primary conductor.
Parallel-Tuned Pickup Control
Continuous (steady) control. Clearly, an absence of switchable control elements is no control at all The saturable inductor of Boys et al as described in NZ329195 (intended for control of overload or fault conditions rather than in normal usage) is also a form of continuous control.
Per-cycle control, where the switching action is timed to occur in a specified relationship with the phase of the circulating current in the resonant pickup, and occurs usually within every cycle:
Turner (assigned to Boeing) in U.S. Pat. No. 4,914,539) (Apr. 3, 1990) describes a regulator circuit in which a 38 kHz current is shorted out for a variable duration per cycle, during a phase-related period following the moment when the voltage passes through zero with a negative slope. This is a regulator for inductively coupled power, for a specific application (aircraft passenger seat entertainment electronics). In the example, semiconductor switching (to cause a shunt) occurs for a controllable period during each cycle. Any excess power is simply shunted to ground. This application exhibits a relatively small variation in load demand. For efficiency reasons this approach is not amenable to scaling upwards, particularly in situations in which the load requirements vary and may go down to zero. The semiconductors are required to work well at high frequency (low reverse recovery time is a desired feature).
Brooks (U.S. Pat. No. 5,045,770 or PCT/AU89/00035) may also be of this type. Brooks describes a shunt circuit, integrated onto a single VLSI chip, for regulating power received from an alternating, loosely coupled, external magnetic field. The regulator shunts input power and includes several modes of operation: diverting excess energy into a load, reducing the Q factor of the pickup circuit, and reducing the power match to the load. A practical circuit includes a synchronous rectifier. This invention is not upwardly scalable.
Series-tuned Pickup Control
Continuous (steady) control has been described by Ehgtesadhi et al, within a number of publications in relation to a variable capacitor serving as the series-tuned capacitor, wherein the capacitor may be switched through 64 steps from zero to slightly beyond the resonant condition, so controlling the output from the more or less tuned pickup.
The saturable inductor of Boys et al as described in NZ329195 (intended for control of overload or fault conditions rather than in normal usage) is also a form of continuous control and could be used in a series-connected resonant circuit.
Per-cycle control, where the switching action is timed to occur in a specified relationship with the phase of the circulating current in the resonant pickup:
Pivnjak and Weiss in Elektrie vol 34 (1980), pp 339 to 341 describe a 5 kHz series-resonant pickup having thyristor switching and (see FIG. 5) phase-related control means, together capable of varying the current circulating within the series-tuned pickup and hence of varying the output.
Lukacs B Nagy I, et al (Proceedings of the 4th Power Electronics conference, Budapest, 1981) also describe at pages 83 to 92, a series-resonant pickup having thyristor switching and phase-related control means, together capable of varying the current circulating within the series-tuned pickup and hence of varying the output.
It is an object of this invention to provide an improved pickup power control system for inductive power transfer or at least to provide the public with a useful choice.
In a first broad aspect the invention provides for an inductive power transfer system, a power pickup device with a series resonant circuit comprising a pickup coil and a resonating capacitor selected so that the pickup is capable of resonance at a system-wide frequency, the power pickup device further including power conditioning means capable of converting electricity that has been picked up into a conditioned form suitable for consumption by a load, wherein apparatus capable of controlling the amount of power picked up by the power pickup device comprises switching means in series with the pickup coil and in series with the resonating capacitor, together with switch controlling means capable of causing the switching means to repetitively be in either an open or a closed state, so that by varying the respective proportion of time that the switching means is either open or is closed the time-averaged amount of power picked up by the power pickup device can be controlled.
Preferably a repetitive cyclic operation of the switching means is relatively slow, so that induced resonating currents may substantially die away during a normal xe2x80x9cOFFxe2x80x9d interval.
Note that switching rate drops as the size of the installation rises, and inductive power transfer installations capable of handling from less than one watt to perhaps one megawatt or more are known Typical repetition rates vary accordingly, from over 1 kHz to less than 100 Hz. Thus it is preferred that the switch control means is capable of providing a repetitive cyclic operation of the switching means which is inversely proportional to the amount of power to be collected by the pickup device.
A preferred switching means comprises a solid-state switch.
Preferably the switching means is a bidirectional switching means capable of controlling an alternating current.
Preferably the switching means is capable of carrying at least a resonating current of a usual magnitude circulating within the pickup.
One preferred solid-state switching means employs the type of device known as an insulated gate bipolar transistor.
A more preferred solid-state switching means comprises a set of inverse parallel fast-recovery thyristors and an example switch device is an asymmetrical silicon-controlled rectifier (ASCR).
Preferably the switch controlling means is capable of responding to the magnitude of the conditioned power in a manner that tends to regulate the magnitude of the conditioned power.
More preferably the switch controlling means is capable of responding to the voltage of the conditioned power
Preferably the switch controlling means is also capable of responding to the instantaneous voltage levels present at each side of the switching means and hence causing the switching means to close at an instant when the the voltage levels present at each side of the switching means are substantially the same.
Preferably the switch controlling means is further capable of detecting the current passing through the switching means and is capable of determining when that current is at a zero crossing point, in order to determine an instant which the switching means may be opened.
The preferred embodiments to be described and illustrated in this specification are provided purely by way of example and are in no way intended to be limiting as to the spirit or the scope of the invention.